Thermal secondary recovery



United States Patent 3,527,303 THERMAL SECONDARY RECOVERY Robert W.Zwicky, Edmonton, Alberta, Canada, assignor to Shell Uil Company, NewYork, N.Y., a corporation of Delaware No Drawing. Filed Mar. 19, 1968,Ser. No. 714,345 Int. Cl. E21b 43/24 US. Cl. 166-303 3 Claims ABSTRACTOF THE DISCLOSURE A steam soak process for the recovery of crude oilwherein a producing formation is first heated with steam and thenproduced. The process includes the adding of caustic or detergents tothe steam to increase the aflinity of the formation rock for water. Whenthe steam condenses in the formation the water is imbibed in the rockinterstices and becomes immobilized. This reduces the water that isproduced when the well is returned to production and increases the oilrecovery.

BACKGROUND OF THE INVENTION The present invention relates to secondaryrecovery processes and more particularly to an improvement in a steamsoak recovery process. The term steam soak is used to refer to arecovery process in which a hydrocarbon-containing formation is heatedby injecting steam into the formation. After a predetermined quantity ofsteam has been injected, the well is normally shut in and allowed tosoak. The length of the soaking period is adjusted so that a substantialquantity of the latent heat of the steam is transferred to the formationto heat the hydrocarbons contained therein to reduce their viscosity andmake them more mobile. After the soaking period, the formationhydrocarbons are produced to the well using a normal depletion reservoirdrainage mechanism as for example, gravity drainage, or solution gasdrive, and removal from the well by pumping or other normal productionmethods.

The steam used in a steam coak recovery method may be high'quality,substantially dry, steam or may be lowquality steam containing aconsiderable amount of water in a liquid phase. A steam soak processusing a low-quality steam is more particularly described and claimed inUS. Pat. No. 3,193,009. The steam may be injected into thehydrocarbon-containing formation for a few hours to several days. Thelength of time that the steam is injected is determined by the viscosityof the oil and the permeability and other formation characteristics thataffect the eflicient transfer of the latent heat of the steam to thehydrocarbons contained in the formation. After the soaking period, thewell is placed back on production using conventional producingmechanisms normally of the depletion type of drive in combination withpumping or smilar lift methods.

A more complete description of a steam soak process as applied torecovering viscous crudes is available in an articles by T. M. Doscherin the Oil and Gas Journal, for July 11, 1966, page 95. While theDoscher article describes the use of a steam soak process as applied tothe recovery of viscous crudes, the process can also be applied toincrease the production of less viscous crudes. The Doscher article alsoprovides data for determining the length of time that steam should beinjected as well as the length of time that the formation should soakbefore the well is placed back on production.

While the above process for steam soaking has been successful inimproving the production from hydrocarboncontaining formations, it doeshave one serious problem.

Patented Sept. 8, 1970 Since the process uses steam to heat theformation, considerable water is produced when the steam condenses.Thus, when the oil from the formation is subsequently produced, thewater is also produced. The production of water is, of course,undesirable since it must be separated from the hydrocarbons before thehydrocarbons can be shipped in a pipeline or by other means. Thus, ifthe amount of water produced could be reduced, the economics of theprocess would be considerably improved.

SUMMARY OF THE INVENTION The present invention solves the problem ofexcessive production of water by adding a chemical to the steam thatwill reduce the surface tension of the water. By reducing the surfacetension of the water, the water will be trapped by the interstices ofthe formation rock and held in place and not produced when the well isreturned to production. In adition to reducing the amount of waterproduced with the hydrocarbons, the trapping of the water in theinterstices of the formation rock will also displace an equivalentamount of hydrocarbons from the formation.

Various types of chemicals may be used for reducing the surface tension,as for example, various caustic compounds or detergent compounds.Various chemicals capable of reducing or lowering the surface tension ofwater are well known to those skilled in the art and any chemical thatis compatible with the formation can be used.

The process of the present invention will only work in particular typesof formation, more particularly, forma tions having ahydrocarbon-containing sand whose natural wettability ranges fromneutral to oil-wet. In addition, the formation in the vicinity of thewell should be free of mobile water, so that the well does not producewater. Likewise, the producing mechanism for the well should beoil-expansion, solution gas drive or gravity drainage, and not asecondary recovery drive means, such as a water drive.

PREFERRED EMBODIMENT As explained above, the present invention isapplicable only to steam soak operations in which a producing formationis heated by injecting steam. After the steam is injected, the well isclosed in and the formation alowed to soak. The well is closed in topermit both the sensible and latent heat of the steam to be transferredto the hy drocarbons contained in the formation. After a sufficienttime, the well is placed back on production and produced by means of aconventional producing mechanism of the depletion type. For example, asolution gas drive or gravity drainage may be used to cause the oil toflow into the well from which it may be removed by pumping or other liftmeans.

In addition to the above limitations of the steam soak process, theinvention is also limited to formations that are capable of oilproduction and having a natural wettability ranging from neutral tooil-wet. The invention will not operate where the formation has beensubject to a secondary recovery employing water flood or where theinitial primary production was the result of a water drive. In addition,the formation in the vicinity of the well is preferably free of mobilewater so that the producing of the well does not normally produce water.

When the above requirements are met, the process of this invention maybe economically employed. The invention is best applied in the case of asteam soak process using a low-grade steam or at least a steamcontaining some liquid phase. This will permit the chemicals to beconveyed to the formation with the liquid phase. A process employinglow-grade steam is described in the abovereferenced patent. Chemicalsthat are capable of increasing the water-holding capacity of anoil-bearing formation by altering the wettability of the formation areinjected with the liquid phase of the steam. A substantially lowconcentration of the chemicals, as for example, approximately 4milligrams of sodium hydroxide per liter of steam, is used. Suitablechemicals for use are the various caustic compounds such as sodiumhydroxide, and nonionic detergents or surfactants such as poly-glycolethers or aryl-alkyl-sulfonates. Additional chemicals are known to thoseskilled in the secondary recovery art and have been used in variousWater flood processes to increase the scouring effect of the water as itis moved through the formation.

After the steam is injected and the well is shut in, a portion of thesteam will condense and the treated water will increase the waterwettness of the formation. This will increase the portion of the waterthat is held by the formation and prevent its production when the wellis returned to production.

The invention is particularly useful in a steam soak process employing aone-well system producing a viscous crude, as explained above. In aone-well system, the invention has the following effects:

(1) Injecting the treated steam into a closed reservoir raises the oilproduction potential of the reservoir by retaining a portion of thewater in the reservoir formation. The amount of water retained in thereservoir formation will at the same pressure displace an equivalentVolume of crude from the reservoir. Thus, the production rate of thecrude as a result of the injection will be above the pre-injection rate.

(2) The heat economy of the process will be improved using treatedsteam, since the condensed steam will be held in the formation. Thus, ifthe heated Water is held in the formation, it will transfer anadditional amount of heat to the crude in the formation and enhance thecrude recovery.

While the above description has been related specifically to a preferredarrangement including a steam soak process and a one-well system, it mayunder certain limited conditions be applied to steam soak processesusing multiple wells. In all cases the invention must be limited toformations that have a natural wettability ranging from neutral tooil-wet. Also, any formation that has been subject to a secondaryrecovery process employing a water flood drive is unsuitable for thepractice of the present invention. Except for the above limitations, theinvention can be applied to a multiple well recovery system. The amountof improvement will be proportional to the change towards Water wetnessachieved. The maximum capacity is achieved at maximum or total waterwetness.

I claim as my invention:

1. A method of primary recovering essentially only liquid hydrocarbonfrom an underground earth formation free of having-been subjected to anyprevious water drive oil recovery treatment and having a naturalwettability ranging from neutral to oil-wet penetrated by well borecomprising:

(a) adding to steam an effective amount of caustic to reduce the surfacetension of water but insuflicient to cause emulsification;

(b) injecting the caustic containing steam into the formation;

(c) closing the well to allow the formation to steam soak, and

(d) placing the well on production to etfect liquid hydrocarbon recoveryby means selected from the group consisting of gravity drainage andsolution gas drive.

2. The method of claim 1 wherein the caustic is NaOH and the amount usedis approximately 4 mg./liter of steam.

3. The method of claim 2 wherein the liquid hydrocarbon recovery iseffected by a backflow producing technique.

References Cited UNITED STATES PATENTS 2,813,583 11/1957 Marx et a1.16611 3,193,009 7/1965 Wallace et a1. 16611 3,259,186 7/1966 Dietz 166403,279,538 10/1966 Doscher l6611 3,302,713 2/1967 Ahearn et al 166-93,396,792 8/1968 Muggee 16640 STEPHEN J. NOVOSAD, Primary Examiner

